1. Field of the Invention
The invention is based on a method for welding age-hardenable nickel-base alloys in such as workpieces made of age-hardenable nickel-base alloys using a filler material of the same composition.
2. Discussion of Background
Nickel-base alloys contain, in addition to Cr, primarily also Co, Mo and W for increasing the strength of the mixed crystal, and usually Ti and Al for forming creep-inhibiting precipitations in the form of inter-metallic phases (.gamma.'-phase), which make these alloys age-hardenable. Moreover, small quantities of rare-earth metals, such as cerium, hafnium, zirconium and yttrium, are often added for their corrosion-inhibiting effect.
Age-hardenable nickel-base alloys are regarded as difficult to weld and tend to form cracks in the heat-affected zone, but particularly in the weld metal. For this reason, such alloys are only welded at uncritical locations using "amenable" Ni-base alloys, such as for example IN625, which do not have the high strength of the base material. Consequently, workpieces which have been welded in this way have at most the strength of the "amenable" Ni-base alloy.
Thus precision castings made from age-hardenable Ni-base alloys, such as for example IN738LC, have to be scrapped in the event of casting defects, such as inclusions, nests of pores, etc., since these defects cannot be eliminated by welding, because according to the above method welded precision castings would have unsatisfactory materials properties.
DE-A 30 15 638 has disclosed a method for improving the physical properties of heat-damaged zones of welds carried out on nickel-base alloys. To do this, the welded body is heated and an isostatic pressure is applied, with the result that the material is deformed plastically, the porosity is reduced and the heat-damaged zone is homogenized. The body is heated in an inert or oxygen-free atmosphere. The welding is carried out with filler and base materials of the same composition. The drawback of this method consists in the fact that only those of the cracks and pores which are not in communication with the surface are closed up by the above-described treatment. This is generally unsatisfactory for the component to function.